Mycorrhizal symbiosis and seedling performance of the frankincense tree (Boswellia papyrifera)

Arid areas are characterized by a seasonal climate with a long dry period. In such stressful environment, resource availability is driven by longterm and shorterm rainfall pulses. Arbuscular Mycorrhizal (AM) fungi enhance access to moisture and nutrients and thereby influence plant performance. In this dissertation I applied field observations and greenhouse experiments to address four questions: 1) What are the major environmental factors influencing AM incidence in the Boswellia-dominated dry deciduous woodlands? 2) How do Boswellia seedlings respond when they are exposed to AM fungi and water pulses? 3) How do AM fungi, water deficit and soil fertility influence the growth and gas exchange of Boswellia and Acacia seedlings? 4) Does the AM symbiosis influence competition between Acacia and Boswellia seedlings at different water pulse levels? The present study showed that almost all woodland plants in northern Ethiopia are colonized by AM fungi. Root colonization levels in dry and wet seasons demonstrated that in the sites with the harshest conditions, AM plants and fungi respond to pulsed resource availability by temporally disconnecting carbon gain by the plant and carbon expenditure by the fungus. Consequently, we studied below-ground processes in conferring adaptation to highly pulsed resources in Boswellia seedlings. The strong interactive AM fungi and water pulse showed that mycorrhizal Boswellia benefits from drought pulses during the short rainy season. Boswellia acquires carbon and water after rain events and store probably carbon and water in coarse roots, suggesting conservative strategy. From this observation we carried out an experiment to test whether other trees (Acacias) than Boswellia in this habitat also show this conservative acquisition strategy, or whether more acquisitive strategies may also be beneficial under such climates. My study show that acquisitive and conservative species both benefit from the AM symbiosis, but that the acquisitive Acacias mainly benefit at higher water availability, whereas the conservative Boswellia benefits at water or nutrient-stressed conditions. I also investigate on how mycorrhiza and water availability affect competition between plants with different resource acquisition strategies in these drylands. Seedlings of Boswellia are competitively inferior to seedlings of Acacia, and neither the presence of AM fungi nor a stronger water limitation (through pulsing) affected this outcome.